African American (AA) women have a higher incidence of breast cancer at young ages and higher mortality from breast cancer at all ages than white women, but are understudied with regard to the genetic etiology. Epidemiologic studies of the genetics of breast cancer have largely focused on European ancestry (EA) populations. Under the current grant, we successfully obtained mouthwash-swish saliva samples from ~27,000 Black Women's Health Study (BWHS) participants, including 1,200 breast cancer cases. We used DNA from breast cancer cases and matched controls to genotype a dense set of tagSNPs for genetic loci that had been associated with breast cancer risk in genome-wide association scans (GWAS) of EA or Asian ancestry populations. In two of those regions, 5p12 and 16q12, we identified new SNPs associated with breast cancer in AA women. We also carried out fast-track replication of the top 5 SNPs from the only collaborative AA GWAS of breast cancer yet conducted, thus helping to identify a novel SNP. In this competing continuation, we propose to follow up our BWHS findings by targeted resequencing of the 5p12 and 16q12 regions in germline DNA from 50 BWHS subjects to identify all other mutations in the regions; we will then assess associations with each of the new variants in our entire set of cases and controls. We will increase the power of the collaborative AA GWAS to discover variants associated with breast cancer by genotyping the 1,500 SNPs with lowest p-values in Stage 1 in a nested case-control study of 1,200 case-control pairs within the BWHS. We will then combine these findings with our observational data to investigate gene-environment interactions with important anthropometric and reproductive factors in 1,100 incident breast cancer cases and 2,200 controls. Many AA women are vitamin D deficient. In another aim, we will examine the relation of vitamin D to incidence of breast cancer in the 1100 incident cases and their 2200 matched controls. A prediction model for vitamin D based on BMI, physical activity, latitude and dietary and supplemental vitamin D intake has proved useful in EA studies of vitamin D and cancer. We propose to extend the model, which would be inadequate in AA women because of the major role of skin pigmentation in determining vitamin D levels, by adding data on skin pigmentation genes and vitamin D receptor and binding protein genes. The prediction model will be developed and validated based on 520 blood samples from BWHS participants. In our final aim, we will use BWHS samples to improve power of the first yet AA GWAS of age at menarche to discover variants associated with this phenotype by genotyping in a sample of 3000 BWHS participants the 1,500 tagSNPs that achieved the highest significance levels in a Stage 1 AA GWAS of age at menarche. Thus, the proposed study will aid discovery of genetic variants associated with breast cancer and age at menarche in AA women, will carry out the first assessment of important potential gene-environment interactions in AA women, and will provide novel data on the association of vitamin D with breast cancer incidence in AA women.